The last minutes of his life were spent lying in Jess' arms, his children near him in the car.
"I had 20 minutes holding him and telling him how loved he is and that we are okay," she wrote in a heartbreaking post on Facebook just days after the tragedy.
"We are in a state of shock, disbelief and absolute devastation."
She went on to write she was changing her young son Edward's name to honour his father.
James' life was celebrated at an Adelaide funeral yesterday and his youngest child will be renamed in his honour.
His widow told mourners the "terror" of the accident would haunt their family forever.
"However he knew his little family were okay. We were there with him. He knew he was loved so deeply by everyone and he knew that this was not his fault."
Walking away from the car last week, "everything flooded from me except my love for him and the kids. Nothing else matters."
As a dad, James was the "cream of the crop", Jess said, adding that their two children were incredibly lucky to have had him.
"My life is now dedicated to making sure that they have the same values as their father."
A death notice for James features the names of his children, Chloe and "little Jimmy".
"For those who know him well, you will know that James was the most hands-on father out there. They are grieving hard too," Jess wrote on Facebook.
"He died way too young and at the peak of his life."
James had excelled at singing, sports, academia and performance arts and won many awards while attending Kings' Prep School and Kings' College, his mother Susan said.
"It's so hard to say goodbye to him ... He was an absolute blessing and an absolute gift that has been taken from us way way too soon."
James had moved to Hong Kong in 2010, and there met Jess, later to become his wife. The pair locked eyes across a room - the next day Jess told her mother she had met her future husband.
The young couple owned and ran a charter yachting company as a husband-wife team in Hong Kong, while James also worked in logistics and supply chain management.
They had bought two more boats to add to the fleet and were due to sail one from Adelaide to Hong Kong at the end of February.
Next month was to be their fifth wedding anniversary.